A fuel cell comprising a polymer electrolyte membrane generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. This fuel cell is basically composed of a polymer electrolyte membrane for selectively transporting hydrogen ions and a pair of electrodes formed on both surfaces of the polymer electrolyte membrane, i.e., an anode and a cathode. The electrode usually comprises a catalyst layer which is composed mainly of carbon particles carrying a platinum group metal catalyst and is formed on the surface of the polymer electrolyte membrane and a diffusion layer which has both gas permeability and electronic conductivity and is formed on the outer surface of the catalyst layer.
In order to prevent the fuel gas and oxidant gas supplied to the electrodes from leaking out or prevent these two kinds of gases from mixing together, gaskets are arranged on the periphery of the electrodes with the polymer electrolyte membrane therebetween. The gaskets are normally composed of a rubber or an elastomer having high chemical resistance such as EPDM rubber, silicone elastomer and fluoro-elasomer. The gaskets are combined integrally with the electrodes and polymer electrolyte membrane beforehand. This is called “MEA” (electrolyte membrane-electrode assembly). Disposed outside the MEA are conductive separator plates for mechanically securing the MEA and for connecting adjacent MEAs electrically in series, or in some cases, in parallel. The separator plates have a gas flow channel for supplying a reaction gas to the electrode surface and for removing a generated gas and an excess gas at a portion to come in contact with the MEA. Although the gas flow channel may be provided separately from the separator plates, grooves are usually formed on the surfaces of the separator plates to serve as the gas flow channel.
In order to supply the fuel gas and oxidant gas to such grooves, it is necessary to use piping jigs which branch respective supply pipes for fuel gas and oxidant gas, depending on the number of the separator plates to be used, and connect the branches directly to the grooves of the separator plates. This jig is called “manifold”, and the above-described type, directly connecting the supply pipes for fuel gas and oxidant gas with the grooves, is called “external manifold”. A manifold having a simpler structure is called “internal manifold”. In the internal manifold, the separator plates with the gas flow channel formed thereon are provided with through holes which are connected to the inlet and outlet of the gas flow channel such that the fuel gas and oxidant gas are supplied directly from these holes.
Since the fuel cell generates heat during operation, it needs cooling with cooling water or the like to keep the cell under good temperature conditions. Normally, a cooling section for flowing the cooling water therein is formed every one to three cells. The cooling section is inserted between the separator plates in one structure and the cooling section is formed by providing the backsides of the separator plates with a cooling water flow channel in the other structure, and the latter is often employed. In a general structure of the fuel cell, the MEAs, separator plates and cooling sections are alternately stacked to form a stack of 10 to 200 cells, and the resultant stack is sandwiched by end plates with current collector plates and insulating plates and is clamped with clamping bolts from both sides.
A serious problem in this type of fuel cells is gas cross leakage. FIG. 44 illustrates the structure in the vicinity of a manifold aperture for the oxidant gas in a fuel cell having the above-described structure. An MEA, composed of a polymer electrolyte membrane 1, an anode 5 and a cathode 7 sandwiching the polymer electrolyte membrane 1 therebetween, and two pieces of gasket 3 arranged on the outer circumference of these members, is sandwiched between two conductive separator plates 4. A flow channel 8 for the oxidant gas is formed in one surface of each of the conductive separator plates 4, while a flow channel 6 for a fuel gas is provided in the other surface thereof. Accordingly, in the gas flow channel section near the manifold aperture 2, the gasket 3 is not supported by the separator plate 4. Therefore, deformation of the gasket, namely hanging down into the gas flow channel 8, occurs. As a result, in two positions, leakage paths running from the anode to the oxidant gas manifold aperture 2 are created. One is a leakage path resulting from the detachment of the gasket from the anode side of the separator plate, and the other is a leakage path caused by the detachment of the gasket from the electrolyte membrane as a result of hanging down of the gasket into the gas flow channel.
For the electrolyte membrane, in general, a perfluorosulfonic acid membrane is used. This is a modified fluorocarbon resin membrane and has poor surface activity and no adhesiveness with respect to the gasket, and therefore the gasket easily detaches from the electrolyte membrane when the gasket deforms. Consequently, when the fuel gas pressure is higher than the oxidant gas pressure, for example, the fuel gas leaks from the anode of a particular cell to the oxidant gas manifold aperture, thereby mixing with the oxidant gas. In the case of a cell stack, the oxidant gas to be supplied to other cell also contains the fuel gas leaked from the particular cell. In other words, gas cross leakage in one cell gives a considerable damage to the overall cell characteristics of the cell stack. Note that while the explanation has been made by illustrating the vicinity of the oxidant gas manifold aperture as an example in FIG. 44, the same explanation can also be given for the vicinity of the fuel gas manifold aperture.
In order to solve the above problem, there is a conventional example in which a bridge 9 for supporting the gasket is provided in the vicinity of the manifold aperture 2 in the gas flow channel 8 so as to prevent the gasket from hanging down. In this technique, however, for each cell, a total of four brides need to be provided in the vicinity of the inlet-side and outlet-side manifold apertures for each of the fuel gas and oxidant gas. In the case of a cell stack, a significant increase in the number of parts is particularly unavoidable, resulting in a problem of very difficult assembly. Moreover, the separator plate needs to be made thicker by an amount corresponding to the thickness of the bridge and that may cause considerable deterioration in the volume power density. A separator plate mainly used nowadays is the one produced by using about a 2 mm thick glassy carbon or stainless steal plate such as SUS316 as the base material, and by forming flow channels by machine work or press molding and then applying various surface treatments for anti-corrosion purposes if the separator plate is made of metal. Since the mean depth of the gas flow channel in the vicinity of the manifold aperture is reduced by the bridge, at least the gas channel in the vicinity of the manifold aperture needs to be dug deeper for compensation. For this reason, it is difficult to implement the above-mentioned structure without increasing the overall thickness of the separator plate. For instance, when the normal separator plate thickness is 2 mm, if the thickness of the separator plate is increased by 1 mm because of the above-mentioned technique, then the volume power density will be lowered to ⅔.
Furthermore, there is a problem of accuracy of processing the separator plates and the brides. In other words, the effect of the above-mentioned bridge structure will be exhibited when the flow channel depth and the bridge height on the separator plate side perfectly agree with each other over the all flow channel paths. In an actual fact, however, it is hard to expect such a precise processing accuracy, and thus it is possible to reduce the amount of gas leakage but impossible to perfectly prevent the leakage.
A further problem is concerning the processing cost of the separator plates. As described above, in a conventional separator plate, the flow channels are formed by cutting the base material by machine work such as milling. Since the machine work is a batch process, it has very poor mass productivity and takes enormous costs. In order to improve these aspects, there has been a tested technique in which, when the separator material is carbon, a small amount of resin material is mixed into the carbon material, and then the mixture is three-dimensionally molded using a technique such as compression molding or injection molding. However, for fuel cells for use in vehicles, particularly, metal separator plates are advantageous because carbon that is a brittle material may be broken by vibration.
Then, in order to solve the above problem concerning the costs, there have been tested techniques in which a porous electrode itself is processed to have the flow channels, or a wavy metal plate or a corrugate fin metal plate is used as a separator plate, and the front and rear surfaces thereof are used as the flow channels. In this case, in order to prevent gas cross leakage, it is necessary to provide the gasket with flow channels for supplying and exhausting a reaction gas to/from the electrode through the manifold apertures. With some flat-surface gaskets, since the gas flow channels are closed, this technique is not available. However, there was no other effective process that can give a three-dimensional structure to a conventional gasket for use in a polymer electrolyte fuel cell.
It is an object of the present invention to solve the above-mentioned problems and to provide a polymer electrolyte fuel cell that achieves a high output density without gas cross leakage.
It is an object of the present invention to provide a polymer electrolyte fuel cell comprising an electrolyte membrane-electrode assembly with a gasket integrally joined on a peripheral portion of an electrolyte membrane.
It is an object of the present invention to provide a method for manufacturing an electrolyte membrane-gasket assembly or an electrolyte membrane-electrode assembly, capable of easily molding a gasket and forming a catalyst layer without damaging the electrolyte membrane in an assembling process, at high productivity.
In addition, it is an object of the present invention to provide a method for manufacturing an electrolyte membrane-gasket assembly or an electrolyte membrane-electrode assembly, capable of solving a problem associated with a method of forming a gasket directly on a polymer electrolyte membrane, i.e., a problem concerning the protection of the polymer electrolyte membrane, and a matching problem between the process of molding a gasket and a process of printing a catalyst layer.